Process for producing silica particles suitable for use as filler for paper

ABSTRACT

The present invention relates to a process for easily and efficiently producing silica particles having a narrow particle size distribution and a high porosity from inexpensive starting materials such as sodium silicate. The silica particles of the present invention can be obtained in the form of a slurry containing them by (1) forming a slurry by mixing first particles difficultly soluble in an alkali and soluble in an acid, with an aqueous alkali silicate solution to form a first slurry containing the first particles, (2) neutralizing the first slurry with a mineral acid to prepare a second slurry containing second particles wherein silica is deposited on the first particles, and (3) adding a mineral acid to the second slurry to dissolve the first particles from the second particles, to prepare a third slurry containing silica particles.  
     When the silica particles of the present invention are used as a filler even in a small amount for paper making, the resultant papers have excellent brightness, opacity, opacity-after-printing, etc.

TECHNICAL FIELD OF THE INVENTION

[0001] The present invention relates to porous silica particlesparticularly suitable for use as a filler for papers, a process forproducing them and the use thereof for producing filler-containingpapers.

TECHNICAL BACKGROUND OF THE INVENTION

[0002] Papers used for printing or writing usually contain, as fillers,inorganic particles of silica, hydrous silicic acid, talc, calciumcarbonate, clay, kaolin, titanium dioxide, etc. and organic particles ofurea/formaldehyde polymers or the like for improving the opticalproperties such as opacity and brightness, smoothness, touch,printability, writing suitability, etc.

[0003] Papers containing the above-described fillers are produced byadding the fillers and other assistants usually used for malting paper,to paper pulp dispersed in water, forming a wet paper from the obtainedstuff with a Fourdrinier paper making machine, twin-wire paper-makingmachine or the like and drying it.

[0004] Recently, the thickness of the paper sheets tends to be reducedto reduce the basis weight thereof. However, particularly when printingpaper sheets are reduced in weight, the opacity of the printed paper(hereinafter referred to as “opacity-after-printing”) is reduced tocause a problem that the printed letters on the reverse side of thepaper sheets are seen through the paper from the surface of the paper.

[0005] Various fillers are usually added to the papers for the purposeof improving the opacity (including the opacity-after-printing) of thepapers.

[0006] Although investigations have been conducted for the purpose ofdeveloping inorganic and organic fillers for improving the opacity,inexpensive fillers having an excellent effect of improving the opacityhave not yet been developed. Further, since the tendency to the furtherreduction in weight is recently increasing, the development of a fillerhaving a higher power of improving the opacity than that of ordinaryfillers is eagerly demanded.

[0007] Among the fillers currently used for improving the opacity,titanium dioxide has only a low power of inhibiting the penetration ofinks, while it is capable of improving the opacity of white papers.Therefore, the improvement in opacity-after-printing is impossible whentitanium dioxide is used. Further, under such conditions that themaximum light scattering capacity can be exhibited, the retention oftitanium oxide in the paper is uneconomically very low.

[0008] Although organic urea/formalin resins have effects of improvingboth opacity-after-printing and opacity of white paper (opacity beforeprinting), each absolute effect is insufficient.

[0009] Hydrous silicic acid is less expensive than the other fillers andit is relatively effective in imparting the opacity-after-printing to apaper by inhibiting the penetration of inks when it is added to a pulpand the paper is made therefrom. However, its effects including that ofimproving the opacity of white paper have not yet reached the expectedlevel.

[0010] As for hydrous silicic acid, it is known that the oil absorption,which is an index of the capacity of preventing ink from the penetrationand greatly contributes to the improvement in theopacity-after-printing, is increased in proportion to the specificsurface area of hydrous silicic acid and can be controlled by changingthe hydrous silicic acid synthesis conditions.

[0011] However, when the specific surface area of hydrous silicic acidis increased to a level higher than that of the ordinary one and thepaper is dried under the same conditions as the paper-making conditions,the filler itself is shrunk to reduce the oil absorption and, as aresult, the improvement in the opacity-after-printing is reduced.

[0012] Amorphous silica particles disclosed in Japanese Patent No.2,604,316 have a high oil absorption and not so large specific surfacearea. However, the effect thereof on the opacity obtained by addingthese particles to paper is not significantly different from that byadding hydrous silicic acid commonly used at present.

[0013] Japanese Patent Unexamined Published Application (hereinafterreferred to as “J. P. KOKAI”) No. Hei 5-301707 discloses hydrous silicicacid of the formula: SiO₂.nH₂O (wherein n is a positive integer) whereinthe cumulative volume is 1.9 to 4.0 cc/g for pores having a pore radiusin the range of 5×10⁴ Å or less, that is at least 0.5 cc/g for poreshaving a pore radius in the range of 3,000 to 4×10⁴ Å, and that is atleast 0.6 cc/g for pores having a pore radius in the range of 100 to1,000 Å. However, the absolute volume for the pores is small because thecumulative volume of pores having a pore radius of 5×10⁴ Å or smaller isup to 4.0 cc/g. In addition, the cumulative volume of the pores having apore radius of 3,000 to 4×10⁴ Å and capable of absorbing the pigment inthe ink and also that of the pores having a pore radius of 100 to 1,000Å and capable of absorbing the vehicle in the ink are yet below thenecessary levels.

[0014] Japanese Patent No. 2,710,529 discloses a hydrous silicic acidfiller for paper making, which is fine, amorphous hydrous silicic acidobtained by the neutralization reaction of an aqueous sodium silicatesolution in the absence of alkali metal salts, and which contains atleast amorphous magnesium silicate as a fine, amorphous metal compound.However, when the amorphous metal compound content is increased forimproving the opacity, the oil absorption is reduced to impair the powerof inhibiting the penetration of the ink and, as a result, theimprovement in the opacity-after-printing is unsatisfactory.

[0015] The diameters of primary particles usually and widely used arevery small. Although the particle diameters are relatively uniform, theyare not in the form of the primary particles, but they form aggregatesas secondary particles in most cases, and the particle diameterdistribution is usually wide when the particles are used. In addition,even when the average particle size is almost equal, the state of thedistribution is different. It is known that when particles of a smalldiameter are thoroughly dispersed in a paper, the contribution of theparticles on the optical properties of the paper is greater than that ofparticles of a large diameter or a small diameter retained in the paperwith a reagent such as a retaining improver.

[0016] However, the particle size of the filler used ranges in a widerange as described above. When such a filler of various particle sizesis added to a paper-making pulp and a paper is made therefrom, theretaining rate (or retention) of the particles of small sizes is usuallyand seriously low, though it varies depending on the paper-makingmachine. For improving the optical properties, the addition rate of thefiller must be increased. However, the improvement in the opticalproperties by merely increasing the addition rate is limited becausewhen the addition rate of the filler is increased, the strength of thepaper is lowered. On the other hand, although the particles of a largesize are retained in the paper, they also have problems that thespecific surface area of them is small and the contribution of them tothe optical properties is only slight. Under these circumstances, it isdesired to easily obtain a filler having a uniform particle size.

SUMMARY OF THE INVENTION

[0017] Therefore, the object of the present invention is to provide aprocess for easily and efficiently producing silica particles having ahigh porosity from inexpensive sodium silicate or the like.

[0018] Another object of the invention is to provide silica particlescapable of imparting excellent brightness, opacity,opacity-after-printing, etc. to a paper obtained by using such particlesas a filler in the paper making.

[0019] Still another object of the invention is to provide a fillercomprising silica particles having an opacity superior to that ofanother filler when they are used in the same amount.

DETAILED DESCRIPTION OF THE INVENTION

[0020] After intensive investigations made for the purpose of attainingthe above-described object, the inventors have found that silicaparticles with a high porosity can be obtained by depositing silica froman aqueous alkali silicate solution on first particles difficultlysoluble in an alkali and soluble in an acid to obtain a slurrycontaining second particles wherein silica is deposited on the firstparticles and then dissolving the first particles from the secondparticles with a mineral acid. The present invention has been completedon the basis of this finding. The inventors have also found that theopacity can be improved by narrowing the range of the particle sizedistribution even when the average particle size is on the same levelor, in particular, by using silica particles having a very uniform sizeand a particle size distribution very close to a single peak. Thepresent invention has been completed on the basis of these findings.

[0021] The detailed description will be made on the present invention.

[0022] The silica particles of the present invention can be obtained inthe form of a slurry thereof by the following steps:

[0023] (1) mixing first particles difficultly soluble in alkalis andsoluble in acids with an aqueous alkali silicate solution to form afirst slurry containing first particles;

[0024] (2) neutralizing the first slurry with a mineral acid to preparea second slurry containing second particles wherein silica is depositedon the first particles; and

[0025] (3) adding a mineral acid to the second slurry particles todissolve the first particles from the second particles, to prepare athird slurry containing silica particles.

[0026] The silica particles of the present invention have a volume of4.0 to 6.0 cc/g, preferably 4.0 to 5.5 cc/g, for pores having a diameterof 10⁵ Å or less, and that of 2.0 cc/g or more for pores having adiameter in the range of 6,000 to 8×10⁵ Å, and that of 1.0 cc/g or morefor pores having a diameter in the range of 200 to 2,000 Å.

[0027] When the volume is less than 4.0 cc/g for the pores having adiameter of 10⁵ 521 or less, the oil absorption is small and the desiredopacity-after-printing cannot be easily imparted to the paper. On thecontrary, the silica particles having a volume of larger than 6.0 cc/gfor the pores having a diameter of 10⁵ Å or less cannot be easilyproduced. As for the pores having a diameter in the range of 6,000 to8×10⁴ Å, when the volume of them is less than 2.0 cc/g, a paper-makingfiller having a high ink absorption cannot be easily obtained becausethe absorption of a pigment component in the ink, absorbable in thepores having diameters of this range, is poor. When the volume of thepores having a diameter in the range of 200 to 2,000 Å is below 1.0cc/g, the absorption of the vehicle, among the ink components, isinsufficient for obtaining the paper-making filler having excellent inkabsorption and effect of preventing strike-through of ink to the reverseside of the paper (hereinafter referred to as “non-strike througheffect”). For obtaining a paper having a high ink absorption, the volumeof pores having a diameter in the range of 200 to 2,000 Å is desirably1.0 to 2.0 cc/g.

[0028] The pore volume was determined with a mercury porosimeter (type:Poreosizer-9320; a product of Micro Meritics). As for the volume forpores having a diameter of 10⁵ Å or less, that for pores having adiameter of 12 to 10⁵ Å was determined.

[0029] The particle size distributions of the first particles and silicaparticles were determined with a laser diffraction particle sizedistribution determination device (type: SALD-2000 J; a product ofShimadzu Corporation). This method will be referred to as “laser method”hereinafter.

[0030] The oil absorption of the silica particles of the presentinvention is 300 to 500 ml/100 g, preferably 350 to 500 ml/100 g. Whenthe oil absorption is below 300 ml/100 g, the opacity-after-printingcannot be easily imparted to the paper, and the silica particles havingan oil absorption of above 500 ml/100 g cannot be easily produced. Theoil absorption was determined according to JIS K 5101.

[0031] The bulk specific gravity of the silica particles of the presentinvention is 0.1 g/ml or below, preferably 0.09 g/ml or less (the lowerlimit is usually 0.06 g/ml). It is supposed that when the bulk specificgravity of the silica particles is as low as 0.1 g/ml, the volume ofthese particles in a paper will be large when they are added to thepaper to make a large contribution to the ink absorption. When the bulkspecific gravity is above 0.1 g/ml, such an effect is difficultlyexhibited. The bulk specific gravity was determined according to JIS K5101.

[0032] The average particle diameter as determined by the laser methodis 5 to 30 μm, preferably 8 to 25 μm, and the standard deviation of theparticle volume distribution to the particle diameter (μm) representedby the logarithm is in the range of 0.1 to 0.25, preferably 0.1 to 0.2.

[0033] When the amount of the silica particles in the paper is fixed,the smaller the average particle diameter, the higher the opacity.However, when the average silica particle diameter is lower than 5 μm, alarge amount of the particles is necessitated because the retention ofthe added silica particles in the paper is seriously lowered. Althoughan inorganic or organic retention aid can be added for the purpose ofincreasing the retention, the amount thereof is limited because a largeamount thereof impairs the formation of the paper. On the contrary, whenthe particle diameter is larger than 30 μm, the number of the particlescontained in the paper is reduced to also reduce the light-scatteringeffect of the silica particles and thereby to reduce the opacity of thepaper, though the retention thereof in the paper is extremely high.

[0034] Another characteristic of the silica particles used in thepresent invention is that the particle diameter distribution can becontrolled in such a range that the standard deviation of the particlevolume distribution to the particle diameter (μm) represented bylogarithm is in the range of 0.1 to 0.25, preferably 0.1 to 0.2. Sincethe silica particles are in the form of aggregates of single particlesas a secondary particles, as described above, they are actually used inthe form of a mixture of the aggregates of the particles having smalland large particle diameters. Therefore, when they are used as they are,the particles of small diameters are difficultly retained in the waterand, on the contrary, although the particles of large diameters areretained in the paper, they do not effectively contribute to the opacityof the paper. When the standard deviation is higher than 0.25, theamount of the particles having small diameters and those having largediameters are increased, and the function of them as the filler isinsufficient.

[0035] The silica particles having an average particle diameter of 5 to30 μm as determined by the laser method and a standard deviation of 0.1to 0.25 for the particle volume distribution to the particle diameterrepresented by the logarithm are obtained by, if necessary, subjectingthe silica particle-containing slurry obtained as described above to adry or wet pulverization and then classifying the particles into twogroups with a vibrating screen or the like.

[0036] The characteristic values of the first particles (in case theyare determined) and silica particles are those obtained by filtering theslurry containing these particles, washing the particles with water,drying them with a dryer at 105° C. and determining them by theabove-described methods.

[0037] The detailed description will be made on a suitable method ofproducing the silica particles of the present invention.

[0038] The aqueous alkali silicate solution used in step (1) of thepresent invention is not particularly limited, and is preferably anaqueous sodium silicate solution or aqueous potassium silicate solution.As for the molar concentration of the alkali silicates in the aqueoussolution, that of sodium silicate is selected from a molar ratio ofSiO₂/Na₂O in the range of 2.0 to 3.4.

[0039] The first particles to be added to the aqueous alkali silicatesolution in the present invention are those difficultly soluble in analkali and soluble in an acid. The expression “difficultly soluble in analkali” herein indicates that the first particles are not soluble in anaqueous alkali solution of pH 9 or above in a short time, namely in 120minutes.

[0040] The materials for the first particles are not particularlylimited so far as the diameter of the particles can be controlled.

[0041] The first particles include those of metals, metal salts, metaloxides, metal hydroxides and organic materials. Metals include thosemetals belonging to Groups 1A to 7A, 8, 1B and 2B of the Periodic Table,such as K, Rb, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Zn, etc. Metal saltsinclude those salts of the metals such as carbonates and manganates, forexample, calcium carbonate, barium carbonate, magnesium carbonate,nickel carbonate, potassium manganate, etc. Metal oxides include theoxides of these metals such as magnesium oxide, zinc oxide, calciumoxide, manganese oxide, etc. Metal hydroxides include those hydroxidesof the metals such as magnesium hydroxide, calcium hydroxide, manganesehydroxide, etc. These particles may be used singly or as a mixture oftwo or more of them.

[0042] The first particles are mixed with the aqueous alkali silicatesolution to obtain a first slurry containing the first particles. Theamount of these first particles in the first slurry is usually 5 to 120%by weight, preferably 10 to 60% by weight, based on the solid (in termsof silica) in the aqueous alkali silicate solution.

[0043] Various mineral acids are usable for neutralizing the firstslurry and thereby to precipitate silica on the first particles. Thesemineral acids are usable in the form of a mixture of two or more ofthem. Examples of the mineral acids include hydrochloric acid, sulfuricacid and nitric acid. Sulfuric acid is suitable for use as the mineralacid because it is easily available on the market at a relatively lowcost. The concentration of the mineral acid which is not particularlylimited is usually 10 to 30% by weight.

[0044] The diameter of the first particles is controlled as desiredbefore they are mixed with the aqueous alkali silicate solution. Theaverage particle diameter is in the range of 0.01 to 10 μm, preferably0.1 to 5 μm. When the average particle diameter is larger than 10 μm,the quantity of the first particles to be contained in the secondparticles wherein silica is deposited on the first particles is reducedand, therefore, the particles added are wasted. On the contrary, firstparticles smaller than 0.01 μm in diameter are economicallydisadvantageous because the production thereof necessitates a high costand much labor but the effect thereof in improving the brightness andopacity is insufficient. Further, by making the size of the firstparticles uniform, the particle size distribution of the obtained silicaparticles of the present invention can be made uniform and to have asingle peak.

[0045] Various mineral acids are usable for dissolving the firstparticles in the second particles wherein silica is deposited on thefirst particles. Mineral acids usable in the present invention are thosereactive with the first particles to form salts which are easilyremovable by washing with water or the like. The mineral acids areusable either alone or in the form of a mixture of two or more of them.The concentration of the mineral acids which is not particularly limitedis usually 10 to 30% by weight.

[0046] The first particles are usually added to the aqueous alkalisilicate solution under stirring the solution, or the aqueous alkalisilicate solution may be added to an aqueous slurry of these firstparticles.

[0047] The first particles are added to the aqueous alkali silicatesolution in a period ranging from before the addition of the mineralacid for the neutralization to the precipitation of silica by theaddition of the mineral acid in case the mineral acid is added only onceor two or more times as will be described below. Namely, the order andnumber of times of the addition of the first particles and the mineralacid for the neutralization are not particularly limited so far as theyare added prior to the complete neutralization of the alkali silicate.The addition may be conducted at once, intermittently in small portionsor continuously.

[0048] The amount of the first particles is preferably 5 to 120% byweight, more preferably 10 to 60% by weight, based on the solid content(in terms of silica) in the aqueous alkali silicate solution. The amountof these first particles in this range is desirable from the viewpointof the suitableness as the paper-making filler. When the amount of thesefirst particles is smaller than 5% by weight, the obtained silica fillercannot impart the desired brightness and opacity to the paper. On thecontrary, even when the amount of these first particles exceeds 120% byweight, the excellent brightness and opacity are no more improved. Insuch a case, a larger amount of the mineral acid is necessitated fordissolving the first particles, after the completion of theprecipitation by neutralization to increase the production cost of thesilica filler economically disadvantageously.

[0049] When the mineral acid for neutralizing the aqueous alkalisilicate solution is added at once in the step (2) of the presentinvention, the temperature of the first slurry is 60° C. or higher andnot above the boiling point of the slurry, preferably 70° C. or higherand not above the boiling point thereof, to form the second particleswherein silica is deposited on the first particles. The mineral acid maybe added either at once or continuously.

[0050] Although the boiling point of the first slurry is usually equalto the boiling point of the aqueous alkali silicate solution, it mayvary depending on ions dissolved therein, the pressure of the system,etc. In practice, the boiling point herein indicates the temperature atwhich the liquid in the slurry boils. Usually, the boiling point is inthe range of 95 to 105° C.

[0051] When the mineral acid for neutralizing the aqueous alkalisilicate solution is added dividedly in two or more portions, 10 to 50%,preferably 20 to 40%, of the total amount thereof necessitated forneutralizing the aqueous alkali silicate solution is added first at aslurry temperature of 20 to 60° C., preferably 30 to 60° C. Then, thetemperature is elevated to a range of 70° C. to the boiling point of theslurry to conduct the aging if necessary. In this step, theabove-described amount of the mineral acid can be added at once orcontinuously to the slurry.

[0052] Then, the aqueous alkali silicate solution is heated to atemperature in the range of 70° C. to the boiling point of the slurry,preferably 85° C. to the boiling point of the slurry in a short periodof time such as 10 to 30 minutes, and aged, if necessary. Thereafter,the second portion of the mineral acid is added at once or continuouslyto neutralize the aqueous alkali silicate solution and, further aging isconducted if necessary.

[0053] In the step (3) of the present invention, the mineral acid isfurther added to the second slurry containing the second particles,obtained as described above, to dissolve the first particles from thesecond particles. The mineral acid used for the dissolution can be addedat once, in portions or continuously. The pH of the second slurrycontaining the second particles is controlled at 2 to 6.5, preferably 4to 6.

[0054] The temperature at which the first particles in the secondparticles are to be dissolved is not particularly limited. Namely, thedissolution can be conducted after lowering the temperature to, forexample, 20° C. or without lowering the temperature.

[0055] The amount of the mineral acid to be added in this step is suchthat the whole first particles are dissolved therein.

[0056] In the present invention, an electrolytic substance such assodium sulfate can be previously added so that the viscosity of theslurry is kept low and stable when the formation and aging of the secondparticles are accelerated. The term “aging” herein indicates that theslurry is stirred at a predetermined temperature in the range of, forexample, 60° C. to the boiling point of the slurry for a predeterminedtime such as 10 to 180 minutes.

[0057] The particle size and distribution were determined with theparticle size distribution determination device (Type: SALD-2000 J; aproduct of Shimazu Corporation). There was no peak due to the firstparticles in the second particles and no difference was found betweenthe second particles and the silica particles after the dissolution. Itwas also found that the pore volume after the dissolution was largerthan that before the dissolution. From this fact, it is supposed thatthe second particles contain the first particles therein in the step offorming the second particles, and that by dissolving the first particleswith the mineral acid, the porosity of the particles is increased.

[0058] Namely, supposedly, silica is deposited on the surfaces of thefirst particles so that silica surrounds the first particles, at leastpartially or substantially completely. The silica layer can beapparently continuously deposited or, alternatively, the fine primaryparticles can be aggregated together to form secondary particles whichform the silica layer. Further, after the dissolution of the firstparticles, a part of these first particles may possibly remain and isadsorbed on the porous silica surfaces.

[0059] In the present invention, the silica particles are obtained inthe form of a slurry thereof, and well-known means and equipment areusable without any change for the transportation and storage of them. Ifnecessary, the silica particles obtained by the present invention may besubjected to the wet grinding and/or wet classification before they areadded to the papers. The means for the wet pulverization includewell-known continuous homomixer, colloid mill, disc refiner, sandgrinder, ball mill, rod mill, etc. When the silica particles are to beclassified after the grinding, they are classified by wet method with aclassifying machine such as a well-known vibrating screen to removecoarse particles larger than 70 μm. The silica particles obtained afterthe above-described treatment has an average particle diameter in therange of 5 to 30, m, preferably 6 to 25 μm and a standard deviation ofthe particle volume distribution to the particle diameter represented bythe logarithm of the particle diameter (μm) in the range of 0.1 to 0.25,preferably 0.1 to 0.2. When the silica particles thus having a narrowparticle size distribution and large pores therein are used as apaper-making filler, an excellent opacity-after-printing can be obtainedeven when the paper sheets are thin and the amount of the filler is notso large.

[0060] As a matter of course, the wet grinding and wet classificationare not always necessary when the diameter of the obtained particles arein this range.

[0061] The silica particles of the present invention have a specificsurface area of 30 to 200 m²/g, preferably 60 to 180 m²/g as determinedby the mercury porosimetry. When the specific surface area is smallerthan 30 m²/g, it is difficult to obtain an oil absorption of 300 ml/100g or more. On the contrary, when the specific surface area exceeds 200m²/g, the properties of the silica particles become like those of a gel,the shrinkage by drying is increased and the oil absorption is inclinedto be low.

[0062] Papers obtained by incorporating the silica particles of thepresent invention as a filler into a pulp material and making the papersfrom the resultant mixture have a high opacity, particularly a highopacity-after-printing. A reason therefor is considered to be that sincethe pore volume in the silica particles is increased to increase the oilabsorption, the capacity of inhibiting the ink from the penetration intothe paper after the printing is increased.

[0063] The silica particles of the present invention are usable as afiller to be dispersed in pulp fibers used as a starting material forpaper in any of acid paper making method, neutral paper making method oralkaline paper making method, or as a pigment for surface coatingagents.

[0064] The silica particles in the form of the slurry produced by theabove-described process can be mixed with a starting material for paperand the obtained mixture can be used for making a paper with a wetpaper-making machine; or the silica particles produced by theabove-described process can be dried and kept in the form of a powder tobe dispersed again in water and mixed with the starting material forpaper.

[0065] The amount of the silica particles used as the filler variesdepending on the desired ash content of the paper and is usually 1 to30% by weight, preferably 1 to 20% by weight.

[0066] The pulps used for preparing papers containing the silicaparticles of the present invention as the filler are known, ordinarypaper-making pulps. They include chemical pulps such as sulfite pulps,craft pulps and soda pulps; wood pulps such as semichemical pulps andmechanical pulps; and non-wood pulps such as paper mulbery, paper bush(Edgeworhia papyrifera) and hemp. These pulps may be either unbleachedpulps or bleached pulps, and either unbeaten pulps or beaten pulps. Theymay be used either alone or in the form of a mixture of two or more ofthem.

[0067] The silica filler-containing paper of the present invention maycontain other fillers than the silica particles of the present inventionand also other additives usually used for the paper making such as asizing agent, defoaming agent, slime-controlling agent, dye, coloringpigment, fluorescent dye, dry strength additive, wet strength additive,drainage aid and retention aid, if necessary.

[0068] The surfaces of the papers containing the silica filler of thepresent invention can be coated with a starch, polyvinyl alcohol,polyacrylamide, surface sizing agent, etc.

[0069] The wet paper making machine used in the present invention issuitably selected from well-known, commercial-scale paper makingmachines such as a cylinder paper machine, inclined former, Fourdriniermachine and twin-wire paper machine depending on the purpose.

[0070] As described above, the silica particles of the present inventionhave a high oil absorption because the pore volume in each particle wasincreased while the specific surface area thereof was kept so that aserious shrinkage is not caused when the silica particles in the form ofa slurry are directly dried, and when the particles are used as thefiller in the paper making, a paper having an excellentopacity-after-printing can be obtained.

EXAMPLES

[0071] The following Examples will further illustrate the presentinvention, which by no means limit the scope of the present invention.In the Examples, percentages are given by weight.

Example 1

[0072] 240 g of commercially available JIS No. 3 aqueous sodium silicatesolution (a product of Tokuyama, solid concentration: 30%) was dilutedwith pure water to 1,000 g. The silica (silicon dioxide) concentrationwas 72 g/kg. The diluted solution was fed into a two-liter stainlesssteel beaker, and 17.9 g of anhydrous sodium sulfate was added theretoat 50° C. Then, 180 g of an aqueous magnesium hydroxide dispersion(#200, a product of Konoshima Kagaku Kogyo, solid concentration: 8%)having an average particle diameter controlled at 0.5 μm with a sandgrinder was added as the first particles difficultly soluble in analkali and soluble in an acid. 54 g (30% based on the whole amount ofacid necessitated for neutralizing sodium silicate) of sulfuric acid(concentration: 20%) was continuously added over a period of 12 minutesunder stirring with Three-One motor.

[0073] After the completion of the addition of sulfuric acid, thetemperature was elevated to 90° C. under stirring in a period of 25minutes. The stirring was conducted at that temperature for 10 minutesto conduct the aging. Then, 126 g of sulfuric acid was continuouslyadded for 23 minutes and the aging was conducted for additional 20minutes. 110 g of sulfuric acid was continuously added for 15 minutes todissolve magnesium hydroxide. The pH of the slurry in this step was 5.2.

[0074] The slurry containing the reaction product was passed through a200-mesh sieve to remove the residue. The average particle diameter ofthe obtained silica particles was 21.3 μm as determined by theabove-described laser method. The slurry passed through the sieve wasfiltered through a Buchner funnel to obtain silica particles in the formof a cake. A part of the cake was dried at 105° C. overnight, and theoil absorption, specific surface area, pore volume and bulk specificgravity of the particles were determined. The balance was dispersedagain in water and stirred to obtain a slurry thereof having aconcentration of 8%. This slurry was used as the paper-making fillerslurry in the following step.

[0075] 25 g (absolute dry weight) of a mixed pulp comprising 15% ofsemi-bleached soft wood kraft pulp, 34% of a thermomechanical pulp(TMP), 11% of mechanical pulp (GP) and 40% of deinked pulp (DIP)obtained from waste newspapers was dispersed in tap water and thedispersion was diluted to a volume of 2 liters to obtain a 1.25% slurry.The filler slurry obtained as described above was added to this slurryin such amounts that the filler content would be 3% based on theabsolute dry weight of the pulp. After stirring for 2 minutes, 1%, basedon the absolute dry weight of the pulp, of aluminum sulfate[Al₂(SO₄)₃.18H₂O] was added and the resultant mixture was stirred for 2minutes. The whole mixture was diluted to 12.5 liters and thoroughlystirred. A paper having an absolute dry weight of 40 g/m² was made witha square sheeting machine (a product of Tozai Seiki) and dried.

[0076] The moisture of the hand-made sheet was controlled in a roomhaving a relative humidity of 65% at 20° C. and the sheet was passedthrough an experimental machine calender (a product of Kumagai RikiKogyo) under a linear pressure of 40 kg/cm twice to control thesmoothness. Then, the paper quality tests for determining the ISObrightness and opacity and the printing tests were conducted by thefollowing methods to evaluate the paper sheet:

[0077] (1) Brightness: The brightness of the paper was determinedaccording to JIS P 8148 (ISO 2470).

[0078] (2) Opacity of white paper: The opacity of white paper wasdetermined according to J. TAPPI 53 (ISO 2471).

[0079] (3) Opacity after printing: The solid printing was conducted withan offset ink for newspapers and an RI printing tester, and theopacity-after-printing Y (%) was defined by the following formula (1):

Y(%)=A/B×100

[0080] wherein A represents the reflectance of the reverse side of apaper after printing, and B represents the reflectance of the reverseside of the paper before printing.

[0081] (4) Retention of silica particles in paper:

[0082] The ash content (A1) of a paper free of silica particles and theash content (A2) of a paper containing silica particles were determinedaccording to JIS P 8128, and the yield was calculated according to theformula:

[(A2)−(A1)]/(addition rate of silica particles in sheet-making step)

Example 2

[0083] 600 g of a 12% aqueous solution of magnesium hydroxide(controlled at 0.5 μm) was added to 240 g of an aqueous solution of No.3 sodium silicate, and the resultant mixture was diluted with pure waterto a volume of 1,000 g. 27 g of sulfuric acid and then 153 g thereofwere added thereto to conduct the reaction. Then, the reaction andtreatment were conducted in the same manner as that of Example 1 exceptthat the amount of sulfuric acid used for dissolving magnesium hydroxidewas altered to 550 g. The obtained slurry containing the silicaparticles was evaluated in the same manner as that of Example 1.

[0084] After the completion of the reaction, the pH of the slurry was4.2, and the average diameter of the obtained particles was 15.6 μm.

Example 3

[0085] The reaction and treatment were conducted in the same manner asthose of Example 2 except that the amount of the aqueous magnesiumhydroxide solution to be added to the aqueous sodium silicate solutionwas altered to 240 g, the amounts of sulfuric acid used in the first andthe second steps were altered to 63 g and 117 g, respectively, and theamount of sulfuric acid used for dissolving magnesium hydroxide wasaltered to 220 g. The obtained slurry containing the silica particleswas evaluated in the same manner as that of Example 1.

[0086] After the completion of the reaction, the pH of the slurry was4.4, and the average diameter of the obtained particles was 19.7 μm.

Example 4

[0087] The reaction and treatment were conducted in the same manner asthose of Example 1 except that the particle diameter of magnesiumhydroxide was altered to 0.1 μm. The obtained slurry containing thesilica particles was evaluated in the same manner as that of Example 1.

[0088] After the completion of the reaction, the pH of the slurry was5.1, and the average diameter of the obtained particles was 18.4 μm.

Example 5

[0089] The reaction and treatment were conducted in the same manner asthose of Example 1 except that the particle diameter of magnesiumhydroxide was altered to 1 μm. The obtained slurry containing the silicaparticles was evaluated in the same manner as that of Example 1. Afterthe completion of the reaction, the pH of the slurry was 5.0, and theaverage diameter of the obtained particles was 23.3 μm.

Comparative Example 1

[0090] The reaction and treatment were conducted in the same manner asthose of Example 1 except that sulfuric acid for dissolving magnesiumhydroxide was not added. The obtained slurry containing the filler wasevaluated in the same manner as that of Example 1.

[0091] After the completion of the reaction, the pH of the slurry was9.3, and the average diameter of the obtained particles was 20.8 μm.

Comparative Example 2

[0092] The reaction and treatment were conducted in the same manner asthose of Example 1 except that magnesium hydroxide was not added, theamount of sulfuric acid added at the first time was 72 g and sulfuricacid for dissolving magnesium hydroxide was not added. The obtainedslurry containing the filler was evaluated in the same manner as that ofExample 1.

[0093] After the completion of the reaction, the pH of the slurry was4.3, and the average diameter of the obtained particles was 19.9 μm.

Referential Example 1

[0094] For comparison, hand-made sheets were made in the same manner asthat of Example 1 except that the filler was not used, and the productswere evaluated.

[0095] The results of the Examples, Comparative Examples and ReferentialExamples are shown in following Table 1. TABLE 1 Specific Oil surfacePore Opacity (%) absorption area volume Brightness White After ml/100 gm²/g cc/g % paper printing Ex. 1 400  97 5.2 51.6 90.1 86.4 Ex. 2 350 83 4.3 51.5 90.3 86   Ex. 3 450 125 5.5 51.8 90.4 86.6 Ex. 4 400 1114.9 51.5 90.1 86.4 Ex. 5 370 104 4.6 51.7 90.1 86.2 Comp. 230  79 3  52   90.6 84.7 Ex. 1 Comp. 250 160 3.2 51.1 88.9 84.9 Ex. 1 Ref. — — —50.6 88.5 82.3 Ex. 1

[0096] It is apparent from Table 1 that the silica particles obtained bythe present invention are capable of imparting high brightness andopacity, particularly an extremely excellent opacity-after-printing, tothe papers containing them (Examples 1 to 5). On the contrary, when theparticles difficultly soluble in alkalis and soluble in acids are notused at all (Comparative Example 2) or when no filler is used(Referential Example 1), the brightness, opacity of white paper andopacity-after-printing are poor disadvantageously.

[0097] On the other hand, when the particles difficultly soluble inalkalis and soluble in acids are not dissolved (Comparative Example 1),the opacity-after-printing cannot be sufficiently improved because thepore volume in the silica particles is small, though the brightness andopacity of white paper can be improved by the effect of these particlesdifficultly soluble or insoluble in alkalis and soluble in acids andincluded in the silica particles.

Example 6

[0098] 240 g of a commercially available JIS No. 3 aqueous sodiumsilicate solution (a product of Tokuyama, solid concentration: 30%) wasdiluted with pure water to 820 g. The silica (silicon dioxide)concentration was 72 g/kg. The diluted solution was fed into a two-literstainless steel beaker, and 17.9 g of anhydrous sodium sulfate and 180 gof an aqueous magnesium hydroxide dispersion (#200, a product ofKamishima Kagaku Kogyo, solid concentration: 8%) having an averageparticle diameter controlled at 0.5 μm with a sand grinder were added at50° C. 63 g (35% based on the whole amount of acid necessitated forneutralizing sodium silicate) of sulfuric acid (concentration: 20%) wascontinuously added over a period of 13 minutes under stirring withThree-One motor. After the completion of the addition of sulfuric acid,the temperature was elevated to 90° C. under stirring during 25 minutes.The stirring was conducted at that temperature for 20 minutes to conductthe aging. Then, 117 g of sulfuric acid was continuously added for 23minutes and the aging was conducted for additional 20 minutes. 110 g ofsulfuric acid was continuously added for 15 minutes to dissolvemagnesium hydroxide. The pH Of the slurry in this step was 4.9.

[0099] The slurry containing the reaction product was passed through a200-mesh sieve to remove the residue. The average particle diameter ofthe obtained silica particles was 21.3 μm as determined by theabove-described laser method. The filler slurry passed through the sievewas filtered through a Buchner funnel to obtain the filler in the formof a cake. A part of the cake was dried at 105° C. overnight, and theoil absorption, specific surface area, pore volume and bulk specificgravity of the product were determined. The balance was dispersed againin water and stirred to obtain a slurry thereof having a concentrationcontrolled at 8%.

[0100] 25 g (absolute dry weight) of a mixed pulp comprising 15% ofbleached coniferous wood kraft pulp, 34% of a thermomechanical pulp(TMP), 11% of mechanical pulp (GP) and 40% of deinked pulp (DIP)obtained from waste newspaper was dispersed in tap water and thedispersion was diluted to a volume of two liters to obtain a 1.25%slurry. The filler slurry obtained as described above was added to thisslurry in such amounts that the filler content would be 3% based on theabsolute dry weight of the pulp. After stirring for 2 minutes, 1%, basedon the absolute dry weight of the pulp, of aluminum sulfate[Al₂(SO₄)₃.18H₂O] was added, and the resultant mixture was stirred for 2minutes and then diluted to 12.5 liters. After the thorough stiring, apaper having an absolute dry weight of 40 g/m² was made with a squaresheeting machine and dried.

[0101] The moisture of the hand-made sheet was controlled in a roomhaving a relative humidity of 65% at 20° C. and the sheet was passedthrough an experimental machine calender under a linear pressure of 40kg/cm twice to control the smoothness. Then, the printing tests wereconducted by the above-described methods to evaluate theopacity-after-printing.

Example 7

[0102] The reactions and treatments were conducted in the same manner asthat of Example 1 except that 300 g of an aqueous magnesium hydroxidesolution (concentration: 12%) having a particle diameter controlled at0.5 μm was added to 240 g of an aqueous solution of No. 3 sodiumsilicate, that the resultant mixture was further diluted to 1,000 g withpure water, that 45 g of sulfuric acid was added first and then 135 gthereof was added, and that the amount of sulfuric acid used fordissolving magnesium hydroxide was 275 g. The obtained filler slurry wasevaluated in the same manner as that of Example 1.

[0103] After the completion of the reaction, the pH of the slurry was4.5 and the average diameter of the obtained particles was 15.6 μm.

Example 8

[0104] The reactions and treatments were conducted in the same manner asthat of Example 6 except that the amount of the aqueous magnesiumhydroxide solution to be added to the aqueous solution of sodiumsilicate was altered to 180 g, that 54 g of sulfuric acid was addedfirst and then 126 g thereof was added, and that the amount of sulfuricacid used for dissolving magnesium hydroxide was 165 g. The obtainedfiller slurry was evaluated in the same manner as. that of Example 6.

[0105] After the completion of the reaction, the pH of the slurry was5.3 and the average diameter of the obtained particles was 19.7 μm.

Comparative Example 3

[0106] The reactions and treatments were conducted in the same manner asthat of Example 6 except that magnesium hydroxide was not added, theamount of sulfuric acid added first was 72 g, and that sulfuric acid fordissolving magnesium hydroxide was not added. The obtained filler slurrywas evaluated in the same manner as that of Example 6.

[0107] After the completion of the reaction, the pH of the slurry was4.3 and the average diameter of the obtained particles was 19.9 μm.TABLE 2 Oil ab- Bulk Pore volume cc/g Opacity Specific sorption specific6000 ˜ after surface ml/ gravity ≦10⁵ 8 × 10⁴ 200 ˜ printing area 100 gg/ml Å Å 1000 Å % m²/g Ex. 6 450 0.076 5.2 3.26 1.52 86.4 115 Ex. 7 3500.082 4.9 2.33 1.71 86.2 135 Ex. 8 430 0.066 5.5 3.64 1.31 86.6 106Comp. 250 0.115 3.3 1.53 1.56 84.9 164 Ex. 3

[0108] It is apparent from Table 2 that the pore diameters of most ofthe silica particles obtained by the present invention are within thepredetermined range and, therefore, a paper having a remarkably highopacity-after-printing can be obtained by using these particles(Examples 6 to 8).

[0109] On the other hand, when the volume of the pores having diameterswithin the predetermined range is small (Comparative Example 3), theopacity-after-printing is poor unfavorably.

Example 9

[0110] Preparation of Silica Particles

[0111] 480 g of a commercially available aqueous solution of JIS No. 3sodium silicate (a product of Tokuyama, solid concentration: 30%) wasdiluted with water to a volume of 2,000 g. Silicon dioxide (silica)concentration was controlled at 72 g/kg. They were fed into a 5-literstainless steel beaker. 36 g of anhydrous sodium sulfate was addedthereto. The temperature of the aqueous solution was adjusted to 50° C.,and then 350 g of an aqueous dispersion (solid concentration; 8%) ofmagnesium hydroxide (a product of Kamishima Kagaku Kogyo; #200) havingan average particle diameter controlled at 0.5 μm with a sand grinderwas added thereto. 108 g (30% based on the whole amount of sulfuric acidnecessitated for neutralizing sodium silicate) of sulfuric acid (20%)was continuously added for a period of 12 minutes under stirring. Afterthe completion of the addition of sulfuric acid, the temperature waselevated to 90° C. under stirring in a period of 25 minutes and then theaging was conducted at that temperature for 10 minutes. The remainingsulfuric acid (252 g, concentration: 20%) was continuously added for 23minutes. Then, the aging was conducted for 20 minutes at thattemperature.

[0112] 220 g of sulfuric acid (concentration: 20%) was continuouslyadded thereto for 15 minutes to dissolve magnesium hydroxide. The pH ofthe slurry was 5.2.

[0113] The obtained slurry was filtered, the filter cake was washed andthe average particle diameter and the standard deviation of the silicaparticles thus obtained were determined to obtain 20.8 μm and 0.18,respectively. A paper-making slurry having a solid concentration of 8%was prepared from the silica particles thus obtained.

[0114] Method of Making Paper:

[0115] 3% (in terms of the solid) (based on the absolute dry weight ofmixed pulp) of the silica particle slurry was added to a mixed pulpslurry (pulp concentration: 1.2%) comprising 15% of bleached coniferouswood kraft pulp, 35% of a thermomechanical pulp, 10% of groundwood pulpand 40% of deinked pulp obtained from waste newspapers. After stirringfor 2 minutes, 1% of aluminum sulfate was added thereto and the obtainedmixture was stirred for 2 minutes. The slurry thus obtained was dilutedto a solid concentration of 0.5% and used as a stock.

[0116] Sheets having an air-dried basis weight of 43 g/m² were preparedfrom the stock with an experimental square sheeting machine (a productof Tozai Seiki). After cooling, the moisture of the sheets wascontrolled in a room having a relative humidity of 65% at 20° C. for 24hours and the sheets were passed through an experimental machinecalender (a product of Kumagai Riki Kogyo) under a linear pressure of 40kg/cm twice to control the smoothness.

Example 10

[0117] Silica particles were prepared in the same manner as that ofExample 9 except that the amounts of the aqueous magnesium hydroxidedispersion and sulfuric acid (concentration: 20%) for dissolvingmagnesium hydroxide were altered to 180 and 110 g, respectively, andpaper sheets were made therefrom. The average particle diameter andstandard deviation of the obtained silica particles were 26.8 um and0.24, respectively. The obtained paper sheets were evaluated in the samemanner as that of Example 9.

Example 11

[0118] Silica particles were prepared in the same manner as that ofExample 9 except that 72 g (20% based on the whole amount) of sulfuricacid was used first and then 288 g thereof was used, and paper sheetswere made therefrom. The average particle diameter and standarddeviation of the obtained silica particles were 10.2 μm and 0.14,respectively. The obtained paper sheets were evaluated in the samemanner as that of Example 9.

Example 12

[0119] The silica particle slurry (solid concentration: 8%) obtained inExample 9 was treated with a sand grinder (SL-1/2G; a product of AIMEX)to obtain silica particles having an average particle diameter of 12.2μm. The silica particles had a standard deviation of 0.19. The averageparticle diameter was changed, but the standard deviation were not sodifferent from each other.

[0120] The same procedure as that of Example 9 was repeated except thatthe silica particles thus obtained were used as the filler.

Example 13

[0121] Hydrous silicic acid was prepared in the same manner as that ofExample 9 except that the amounts of the aqueous magnesium hydroxidedispersion and sulfuric acid (concentration: 20%) for dissolvingmagnesium hydroxide were altered to 600 g and 380 g, respectively, andpaper sheets were prepared by using the product. The average particlediameter and standard deviation of the obtained hydrous silicic acidwere 15.8 μm and 0.203, respectively. The obtained paper sheets wereevaluated in the same manner as in Example 9.

Comparative Example 4

[0122] Preparation of Silica Particles

[0123] 480 g of a commercially available aqueous solution of JIS No. 3sodium silicate (a product of Tokuyama, solid concentration: 30%) wasdiluted with water to a volume of 2,000 g. Silicon dioxide (silica)concentration was controlled at 72 g/kg. They were fed into a 5-literstainless steel beaker. 36 g of anhydrous sodium sulfate was addedthereto. The temperature of the aqueous solution was adjusted to 50° C.144 g (40% based on the whole amount of sulfuric acid necessitated forneutralizing sodium silicate) of sulfuric acid (20%) was continuouslyadded over a period of 12 minutes under stirring. After the completionof the addition of sulfuric acid, the temperature was elevated to 90° C.under stirring for a period of 25 minutes and then the aging wasconducted at that temperature for 10 minutes. The remaining sulfuricacid (216 g, concentration: 20%) was continuously added for 23 minutes.Then, the aging was conducted for 20 minutes at that temperature. The pHof the slurry was 5.2 (slurry A).

[0124] The slurry A was classified with a 200-mesh sieve. The residue(remaining rate: 22%) on the sieve was ground with a sand grinder andmixed with the particles which had passed through the sieve. Theobtained mixture was filtered and the filter cake was washed. Theaverage particle diameter and standard deviation of the obtained silicaparticles were determined by the laser method to be 25.8 μm and 0.33,respectively. A paper-making slurry having a solid concentration of 8%was prepared from the silica particles.

[0125] Paper sheets were prepared in the same manner as that of Example9 except that the silica particles obtained as described above were usedas the filler. The obtained paper sheets were evaluated in the samemanner as in Example 9.

Comparative Example 5

[0126] The silica particle slurry (solid concentration: 8%) obtained inComparative Example 4 was treated with a sand grinder (SL-1/2G; aproduct of AIMEX) to obtain silica particles having an average particlediameter of 11.8 μm. The silica particles had a standard deviation of0.35. The average particle diameter was reduced by the grinding, and therange of the distribution was slightly widened. The same procedure asthat of Example 9 was repeated except that the silica particles thusobtained were used as the filler. The obtained paper sheets wereevaluated in the same manner as that of Example 9.

Referential Example 2

[0127] Paper sheets were made in the same manner as that of Example 9except that the filler was not used. The obtained paper sheets wereevaluated in the same manner as that of Example 9. The results wereemployed as the standards of the evaluation of those obtained inExamples and Comparative Examples.

[0128] The results of the Examples and Referential Example 2 are shownin Table 3. TABLE 3 Average Opacity particle Standard Retention Bright-after diameter deviation of filler ness Opacity printing μm % % %* %* %*Ex. 9 20.8 0.18 40.1 +1.4 +2.0 +3.7 Ex. 10 26.8 0.24 43.6 +1.4 +2.0 +3.3Ex. 11 10.2 0.14 32.8 +1.5 +1.8 +2.8 Ex. 12 12.2 0.19 33.4 +1.5 +1.9+2.9 Ex. 13 15.8 0.20 35.8 +1.5 +2.0 +3.0 Comp. 25.8 0.33 35.2 +1.4 +2.0+2.4 Ex. 4 Comp. 11.8 0.35 26.7 +1.5 +1.7 +2.0 Ex. 5 Ref. — — — standardstandard standard Ex. 2

[0129] It is apparent from Table 3 that by narrowing the particle sizedistribution of the silica particles, the retention of the silicaparticles in the paper is improved, and the paper sheets excellent inthe opacity, particularly opacity-after-printing, can be obtained(Examples 9 to 13). On the other hand, when the characteristic value(standard deviation) of the silica particles is not within the range ofthe present invention (Comparative Examples 4 and 5), even though theparticle diameter levels are the same, the retention of the silicaparticles in the paper is lower and the degree of the increase in theopacity-after-printing is lower as compared with those obtained when theparticle size distribution is narrow, while the effect of improving theopacity is obtained to some extent.

[0130] Effect of the Invention:

[0131] As described above, silica particles having a narrow particlesize distribution and a high porosity can be easily and efficientlyproduced from inexpensive starting materials such as sodium silicateaccording to the present invention. When these silica particles are usedas a filler in the paper making, paper sheets having excellentbrightness, opacity and opacity-after-printing can be obtained.

What is claimed is:
 1. A process for producing silica particles,comprising the steps of: (1) mixing an aqueous alkali silicate solutionwith first particles difficultly soluble in an alkali and soluble in anacid, so as to form a first slurry containing said first particles; (2)neutralizing said first slurry with a mineral acid to prepare a secondslurry containing second particles wherein silica is deposited on saidfirst particles; and (3) adding a mineral acid to said second slurry todissolve said first particles and form a third slurry containing silicaparticles.
 2. The process of claim 1 , wherein said first particles aremade of one member selected from the group consisting of metals, metalsalts, metal oxides, metal hydroxides and organic materials.
 3. Theprocess of claim 2 , wherein a metal is selected from the groupconsisting of K, Rb, Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni and Zn.
 4. Theprocess of claim 2 , wherein said first particles are made of one memberselected from the group consisting of calcium carbonate, bariumcarbonate, magnesium carbonate, nickel carbonate, potassium manganate,magnesium oxide, zinc oxide, calcium oxide, manganese oxide, magnesiumhydroxide, calcium hydroxide and manganese hydroxide.
 5. The process ofclaim 1 , wherein the amount of said first particles is 5 to 120% byweight based on the solid (in terms of silica) in said aqueous alkalisilicate solution.
 6. The process of claim 1 , wherein the amount ofsaid first particles is 10 to 60% by weight based on the solid (in termsof silica) in said aqueous alkali silicate solution.
 7. The process ofclaim 1 , wherein said first particles have an average particle diameterof 0.01 to 10 μm.
 8. The process of claim 1 , wherein said firstparticles have an average particle diameter of 0.1 to 5 μm.
 9. Theprocess of claim 1 , wherein said third slurry in step (3) has a pH of 2to 6.5.
 10. The process of claim 1 , further including, after step (1),heating said first slurry to a temperature in the range of 70° C. to theboiling point of said slurry.
 11. The process of claim 10 , wherein, instep (2), 10 to 50% of said mineral acid necessitated for neutralizingsaid alkali silicate solution is added to said alkali silicate solutionat 20 to 60° C., then the resultant slurry is heated to a temperature of70° C. or higher, and the balance of said mineral acid necessitated forthe neutralization is added.
 12. The process of claim 1 , wherein saidthird slurry is ground and/or classified by a wet method, after step(3).
 13. A filler-containing paper containing said silica particlesobtained by the process of claim 1 as the filler.
 14. Silica particleshaving a cumulative volume of 4.0 to 6.0 cc/g for pores having adiameter of 10⁵ Å or less, that of at least 2.0 cc/g for pores having adiameter of 6,000 to 8×10⁴ Å, that of at least 1.0 cc/g for pores havinga diameter of 200 to 2,000 Å, an oil absorption of 300 to 500 ml/100 g,and a bulk specific gravity of 0.1 g/ml or less.
 15. The silicaparticles of claim 14 , having a specific surface area of 30 to 200m²/g.
 16. A filler-containing paper containing the silica particles ofclaim 14 as the filler.
 17. A process for producing a filler-containingpaper, comprising adding a slurry of silica particles to a pulp slurryand using the resultant slurry for making a sheet of paper, said silicaparticles having an average diameter of 5 to 30 μm as determined bylaser method and a standard deviation of 0.10 to 0.25 in respect of aparticle volume distribution to a particle diameter (μm) represented bylogarithm.